In vehicular seats, and especially in automotive seat manufacturing, hinged recliner mechanisms are generally well known. Typically, such recliners provide some form of manual locking and unlocking between a side plate pawl with a seat back rack to facilitate making selective incremental adjustments to the inclination of the backrest, relative to the seat cushion.
Typically, incremental rates of adjustment are either 2 degrees or three-quarters of a degree.
Due to the inherent practical limitations of typical mass production techniques, the three-quarter degree incremental adjustment usually employs a spur gear assembly intermediate between the locking pawl and the back support arm rack. The spur gear assembly comprises pinion and ratchet wheel spur gears that are fixed to one another, and are mutually rotatable about a common concentric shaft. The ratio of their respective diameters and teeth sizes is relied upon to achieve the fine incremental adjustment rate.
Although such a spur gear assembly is necessary in order to achieve this fine adjustment rate, their presence contributes significantly to a problem known in the art as "chuck".
The problem of chuck arises inherently in any mechanical seat back device. It is the result of the clearances that are necessarily a part of any device containing parts adapted to undergo relative movement. "Clearance" is the difference in size between mating parts prior to their assembly. Typically the clearance on cylindrical parts may be expressed as radial clearance or diametrical clearance. Positive clearance implies that the inside dimension of the female part is larger than the corresponding outside dimension of the male part. In typical mechanical applications a running fit requires about 0.001 inches of positive radial clearance for a one inch diameter shaft. A clearance of 0.003 inches on a one inch shaft is considered to be a loose fit. In mass production techniques typical of those used in automotive seat back hinge manufacture, positive clearances of 0.008 inches are not at all uncommon.
The amount of clearance typically found in seat back hinges of the type above-described, introduce an element of play that is referred to as "chuck". Note that "chuck" is not the "play" which is introduced through the flexure of any mechanical components. Rather, the term is expressly used and understood in the automobile seat back art to mean the component of play which results from mechanical lost motion due to clearance between interconnected parts, when leverage forces are applied across the seat back hinge.
"Chuck" is a highly undesirable characteristic in automobile seat back hinges, for both safety and aesthetic reasons. In fact, any increase in chuck, from the marketing point of view, is in diametric opposition with the very purpose of providing the much larger variation in possible selectable seat positioning that is achieved with the above-mentioned finer incremental adjustment rates.
The increase in chuck that is occasioned when the spur gear assembly is included in seat back recliner hinges arises partly due to the clearances between the mating teeth of the pawl/ratchet wheel and the pinion/rack, and partly due to the clearances between the spur gear assembly shaft and the holes in the side plate through which the shaft usually extends.
The clearances between the mating gear teeth is known as backlash. Backlash is a necessary characteristic of all mating gear teeth, and is needed to give the gears enough freedom to mesh and release during the relative motion between any two contacting gear faces.
In the past, attempts have been made to minimize the amount of backlash that contributes to chuck in seat back hinges, by mounting the spur gear assembly shaft between two spur gear subassembly side plates. During the manufacture of the recliner hinge, this spur gear subassembly is jammed into forced engagement with the rack on the seat back support arm, in an attempt to minimize the backlash clearance between the respective contacting gear faces. The subassembly side plates are then welded into place on the adjacent hinge side plates to secure the ear faces with a minimum of backlash clearance.
While this reduces the amount of backlash, and therefore has some effect on reducing chuck, it also reduces the operating clearance between the gear faces during seat back adjustment operations. In fact, this solution so increases the amount of friction and binding between the gear faces, as to routinely require that a much larger power spring be used in the recliner. Noisy, rough operation is also a by-product of this approach to the problem, and this too is inconsistent with the marketing of the higher quality, (i.e. more adjustable) seat back recliner features associated with upscale automobiles.
Accordingly, there remains a need in the art for improved vehicular seat back recliners.